1. Field of the Invention
The present invention relates to a screen with an improved luminance uniformity.
2. Description of the Related Art
In recent years, overhead projectors and front projectors have widely been used for presentations in meetings and conferences. Liquid crystal video projectors and moving-image projectors are also becoming widespread for household use. These projectors project light on a screen by modulating light from a light source via, for example, a transmissive liquid crystal panel to form a light image and emitting the light image through an optical system, such as lenses.
For example, a projector that forms color images on a screen includes an illuminating optical system that separates light from a light source into red (R), green (G), and blue (B) and then converges them on a predetermined optical path; a liquid crystal panel that modulates each red, green, and blue flux separated by the illuminating optical system (light valve); and a unit for combining the red, green, and blue fluxes modulated by the liquid crystal panel, a color image combined by the unit being magnified and projected on the screen through a projector lens.
Recently, a projector with a narrow-band RGB light source in which RGB fluxes are spatially modulated by a grating light valve (GLV) in place of a liquid crystal panel has been developed.
Screens that display images from these projectors typically include light diffusion layers that scatter light onto the screens to achieve an excellent viewed image. The light diffusion layers have uniform diffusion properties over the screens and a luminance distribution of bilateral symmetry with the maximum at a scattering angle of 0° for light at an incident angle of 0°, as shown in FIG. 4. Diffusion properties of the diffusion layers are usually represented by the angle of the full width half maximum (FWHM) or the half width half maximum (HWHM) of the maximum luminance, as shown in FIG. 4.
When screen gain is increased, such uniform diffusion properties over the screen cause a large difference in luminance between the center and the periphery of the screen with a bright image in the center and a dark image at the periphery. This is because projected light has an incident angle larger than 0° at the periphery, and is largely reflected away from the viewer.
To overcome such luminance difference on the screen, Japanese Unexamined Patent Application Publication No. 10-142699 discloses a screen that includes a reflective layer having gradually increasing surface roughness from the center to the periphery. The diffusibility of light reflected from the screen gradually increases from the center to the periphery.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-162710 discloses a screen that includes a group of mirrors coupled orthogonally with each other and achieves a uniform luminance distribution by adjustment of the tilt angle of each coupled mirror.
Although the screen that includes the reflective layer with controlled surface roughness has an improved luminance distribution, a reflected-light component having the highest luminance at the periphery of the screen is still directed away from the viewer and many reflected-light components do not contribute to the viewed image. Thus this screen uses projected light inefficiently and it is difficult to improve the overall luminance of the screen.
On the other hand, the screen that includes the coupled mirrors can have a horizontally uniform luminance distribution, but cannot have a vertically uniform luminance distribution because of its structure.